Adjustable chair back



FIG. 1 is a perspective view of an adjustable chair back showing my new design;

FIG. 2 is a rear perspective view thereof;

FIG. 3 is a front elevational view with the arms in folded position;

FIG. 4 is a right side elevational view of FIG. 1;

FIG. 5 is a right side elevational view of FIG. 3; and

FIG. 6 is a top plan view thereof.

The broken lines in all views are for illustrative purposes only. 

The ornamental design for an adjustable chair back, as shown and described. 